Energy storage device, method of manufacturing same, and mobile electronic device containing same

ABSTRACT

An energy storage device comprises a first porous semiconducting structure ( 510 ) comprising a first plurality of channels ( 511 ) that contain a first electrolyte ( 514 ) and a second porous semiconducting structure ( 520 ) comprising a second plurality of channels ( 521 ) that contain a second electrolyte ( 524 ). In one embodiment, the energy storage device further comprises a film ( 535 ) on at least one of the first and second porous semiconducting structures, the film comprising a material capable of exhibiting reversible electron transfer reactions. In another embodiment, at least one of the first and second electrolytes contains a plurality of metal ions. In another embodiment, the first and second electrolytes, taken together, comprise a redox system.

CLAIM OF PRIORITY

This application is a continuation of U.S. patent application Ser. No.13/977,564, filed Jul. 29, 2014 as the national stage entry ofPCT/US2012/034972, filed on Apr. 25, 2012.

CROSS REFERENCE TO RELATED APPLICATION

This application is related to International Application No.PCT/US2010/029821, which has been published as WO 2011/123135, filed onApr. 2, 2010 and assigned to the same assignee to which the presentapplication is assigned.

FIELD OF THE INVENTION

The disclosed embodiments of the invention relate generally to energystorage, and relate more particularly to electrochemical capacitiveenergy storage devices.

BACKGROUND OF THE INVENTION

Modern societies depend on the ready availability of energy. As thedemand for energy increases, devices capable of efficiently storingenergy become increasingly important. As a result, energy storagedevices, including batteries, capacitors, electrochemical capacitors(ECs), (including pseudocapacitors and electric double-layer capacitors(EDLCs) (sometimes called ultracapacitors, among other names)), hybridECs, and the like are being extensively used in the electronics realmand beyond. In particular, capacitors are widely used for applicationsranging from electrical circuitry and power delivery to voltageregulation and battery replacement. Electrochemical capacitors arecharacterized by high energy storage capacity, rapid charge/dischargeability, and large cycle lifetimes, as well as other desirablecharacteristics including high power density, small size, and lowweight, and have thus become promising candidates for use in severalenergy storage applications.

The related case mentioned above (International Application No.PCT/US2010/029821) discloses three-dimensional structures for forminghigh energy density electrochemical capacitors using, for example,porous silicon. In some of the embodiments disclosed there, anelectrochemical process was used to etch pores deep into a siliconstructure and the pores were filled with an electrolyte or with a high-kdielectric material and/or a thin conductive film in combination with anelectrolyte.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed embodiments will be better understood from a reading ofthe following detailed description, taken in conjunction with theaccompanying figures in the drawings in which:

FIGS. 1 and 2 are cross-sectional views of an energy storage structureaccording to embodiments of the invention;

FIG. 2B is a cross-sectional view of an energy storage structureaccording to embodiments of the invention.

FIG. 3 is a depiction of an electric double layer formed within achannel of a porous structure according to an embodiment of theinvention;

FIGS. 4a and 4b are images of, respectively, a surface and across-sectional slice of a porous silicon structure;

FIG. 5 is a cross-sectional view of a portion of an energy storagedevice according to an embodiment of the invention;

FIG. 6 is a schematic representation of a mobile electronic deviceaccording to an embodiment of the invention; and

FIGS. 7 and 8 are flowcharts illustrating methods of manufacturing anenergy storage device according to embodiments of the invention.

For simplicity and clarity of illustration, the drawing figuresillustrate the general manner of construction, and descriptions anddetails of well-known features and techniques may be omitted to avoidunnecessarily obscuring the discussion of the described embodiments ofthe invention. Additionally, elements in the drawing figures are notnecessarily drawn to scale. For example, the dimensions of some of theelements in the figures may be exaggerated relative to other elements tohelp improve understanding of embodiments of the present invention.Certain figures may be shown in an idealized fashion in order to aidunderstanding, such as when structures are shown having straight lines,sharp angles, and/or parallel planes or the like that under real-worldconditions would likely be significantly less symmetric and orderly. Thesame reference numerals in different figures denote the same elements,while similar reference numerals may, but do not necessarily, denotesimilar elements.

The terms “first,” “second,” “third,” “fourth,” and the like in thedescription and in the claims, if any, are used for distinguishingbetween similar elements and not necessarily for describing a particularsequential or chronological order. It is to be understood that the termsso used are interchangeable under appropriate circumstances such thatthe embodiments of the invention described herein are, for example,capable of operation in sequences other than those illustrated orotherwise described herein. Similarly, if a method is described hereinas comprising a series of steps, the order of such steps as presentedherein is not necessarily the only order in which such steps may beperformed, and certain of the stated steps may possibly be omittedand/or certain other steps not described herein may possibly be added tothe method. Furthermore, the terms “comprise,” “include,” “have,” andany variations thereof, are intended to cover a non-exclusive inclusion,such that a process, method, article, or apparatus that comprises a listof elements is not necessarily limited to those elements, but mayinclude other elements not expressly listed or inherent to such process,method, article, or apparatus.

The terms “left,” “right,” “front,” “back,” “top,” “bottom,” “over,”“under,” and the like in the description and in the claims, if any, areused for descriptive purposes and not necessarily for describingpermanent relative positions unless otherwise indicated eitherspecifically or by context. It is to be understood that the terms soused are interchangeable under appropriate circumstances such that theembodiments of the invention described herein are, for example, capableof operation in other orientations than those illustrated or otherwisedescribed herein. The term “coupled,” as used herein, is defined asdirectly or indirectly connected in an electrical or non-electricalmanner. Objects described herein as being “adjacent to” each other maybe in physical contact with each other, in close proximity to eachother, or in the same general region or area as each other, asappropriate for the context in which the phrase is used. Occurrences ofthe phrase “in one embodiment” herein do not necessarily all refer tothe same embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

In various embodiments of the invention, an energy storage devicecomprises a first porous semiconducting structure comprising a firstplurality of channels that contain a first electrolyte and a secondporous semiconducting structure comprising a second plurality ofchannels that contain a second electrolyte. In one embodiment, theenergy storage device further comprises a film on at least one of thefirst and second porous semiconducting structures, the film comprising amaterial capable of exhibiting reversible electron transfer reactions.In another embodiment, at least one of the first and second electrolytescontains a plurality of metal ions. In another embodiment, the first andsecond electrolytes, taken together, comprise a redox system.

Although much of the discussion herein will focus on electrochemicalcapacitors, the “energy storage device” designation explicitlyincludes—in addition to ECs—hybrid electrochemical capacitors (which,like electrochemical capacitors, are discussed in more detail below) aswell as batteries, fuel cells, and similar devices that store energy.Energy storage devices according to embodiments of the invention can beused for a wide variety of applications, including in personal computers(PCs), including desktop and laptop (notebook) computers, tabletcomputers, cell phones, smart phones, music players, servers, otherelectronic devices, automobiles, buses, trains, airplanes, othertransportation vehicles, home energy storage, storage for energygenerated by solar or wind energy generators—especially energyharvesting devices—and many others.

Electrochemical capacitors operate according to principles similar tothose that govern conventional parallel plate capacitors, but certainimportant differences do apply. One significant difference concerns thecharge separation mechanism. For one important class of ECs thistypically takes the form of a so-called electric double layer, or EDL,rather than the dielectric of a conventional capacitor. The EDL iscreated at an interface between an electrolyte and a high-surface areaelectrode by the electrochemical behavior of electrons (or holes) on oneside of the interface and ionic charge carriers on the other side, andresults in an effective separation of charge in spite of the fact thatthe two layers within the double layer are so close together. (Physicalseparation distances are on the order of a single nanometer.) Thus, atypical EDL capacitor may be thought of as storing charge in its EDL.Each layer of the EDL, which is formed when a voltage is applied acrossthe interface, is electrically conductive but the properties of thedouble layer prevent current from flowing across the boundary betweenthem. (The EDL is further discussed below in connection with FIG. 3.)

As is true in conventional capacitors, capacitance in an EDL capacitoris proportional to the surface area of the electrodes and inverselyproportional to the charge separation distance. The very highcapacitances achievable in an EDL capacitor are due in part to the veryhigh surface area attributable to the multi-channel porous structure andto the nanometer-scale charge separation distance attributable to theEDL, which arises due to the presence of an electrolyte, as explainedabove. One type of electrolyte that may be used in accordance withembodiments of the invention is an ionic liquid. Another is anelectrolyte (e.g., Li₂SO₄, LiPF₆) comprising an ion-containing solvent.Organic electrolytes, aqueous electrolytes, and solid-state electrolytesare also possible.

Another class of electrochemical capacitor is the pseudocapacitor,where, in addition to EDL capacitance a different storage mechanism—onethat is Faradaic and not electrostatic in origin—can arise at thesurface of certain types of electrodes. This is typically referred to as“pseudocapacitance,” and is characterized by a charge storage processthat is similar to the operation of many solid-electrode batteries.Typically, one of the electrodes of a pseudocapacitor is coated with atransition metal oxide, a suitable conducting polymer, or a similarmaterial that makes up the active material where charge is stored. Thesematerials can be used with an electrolyte such as a potassium hydroxide(KOH) solution; when the device is charged, the electrolyte will reactwith the material and drive a charge transfer reaction where energy isstored. More specifically, these materials store most of their energythrough highly-reversible surface and near-surface electron transfer(e.g., redox (Faradaic)) reactions, which enable higher power than bulkstorage in conventional batteries due to the fast charge and dischargekinetics.

Hybrid electrochemical capacitors are energy storage devices thatcombine the attributes of ECs and batteries. In one example, anelectrode coated with a lithium ion material is combined with anelectrochemical capacitor in order to create a device that has an EC'srapid charge and discharge characteristics and a battery's high energydensity. On the other hand, hybrid ECs, like batteries, have shorterexpected lifespans than do electrochemical capacitors.

Referring now to the drawings, FIGS. 1 and 2 are cross-sectional viewsof an energy storage structure 100 that will be used to guide an initialdiscussion introducing concepts and structures that will aid in theunderstanding of embodiments of the present invention. As illustrated inFIG. 1, energy storage structure 100 comprises an energy storage device101 and a conductive support structure 102. (In some embodiments supportstructure 102 can be omitted.) Alternatively, as illustrated in FIG. 2,energy storage structure 100 comprises energy storage device 101 and anon-conductive support structure 103.

Energy storage device 101 comprises an electrically conductive structure110 and an electrically conductive structure 120 separated from eachother by a separator 130 that is an electron insulator and an ionicconductor. Separator 130 prevents electrically conductive structures 110and 120 from physically contacting each other, thereby preventing anelectrical short circuit. (In other embodiments, for reasons discussedbelow, a separator is not necessary and can be omitted.)

In some embodiments, at least one of electrically conductive structures110 and 120 comprises a porous structure containing multiple channels,each one of which has an opening to a surface of the porous structure.This feature is a result of a process, described below, used to form theporous structure. As an example, the porous structure may be formedwithin a conductive or a semiconductive material. Alternatively, theporous structure may be formed within an insulating material (e.g.,alumina) that has been coated with an electrically conductive film(e.g., an atomic layer deposition (ALD) conductive film such as titaniumnitride (TiN), tungsten, or ruthenium). In this regard, materials havinggreater electrical conductivity are advantageous because they lower theenergy storage device's effective series resistance (ESR). In theillustrated embodiments, both electrically conductive structure 110 andelectrically conductive structure 120 comprise such a porous structure.Accordingly, electrically conductive structure 110 comprises channels111 with openings 112 to a surface 115 of the corresponding porousstructure and electrically conductive structure 120 comprises channels121 with openings 122 to a surface 125 of the corresponding porousstructure.

Various configurations of energy storage device 101 are possible. In theembodiment of FIG. 1, for example, energy storage device 101 comprisestwo distinct porous structures (that is, electrically conductivestructure 110 and electrically conductive structure 120) that have beenbonded together face-to-face with separator 130 in between. As anotherexample, in the embodiment of FIG. 2 energy storage device 101 comprisesa single planar porous structure in which a first section (electricallyconductive structure 110) is separated from a second section(electrically conductive structure 120) by a trench 231 containingseparator 130. One of the electrically conductive structures will be thepositive side and the other electrically conductive structure will bethe negative side. Trench 231 may separate electrically conductivestructure 110 and 120 along a straight line, but alternatively mayseparate them using a more complex shape such as the meandering spacebetween the fingers of two interdigitated electrodes.

As an example, separator 130 could be a permeable membrane or otherporous polymer separator. In general, the separator prevents thephysical contact of anode and cathode (which could cause an electricalmalfunction in the device) while permitting the transfer of ionic chargecarriers. In addition to polymer separators, several other separatortypes are possible. These include non-woven fiber sheets or othernon-woven separators, liquid membranes, polymer electrolytes, solid ionconductors, glass fiber, paper, ceramic, and the like. In someembodiments, non-woven separators are concentrations of fibers that areeither randomly oriented or are arranged in a directional pattern.

It should be noted that the separator, although shown in FIG. 2, may notbe necessary in the configuration illustrated there because, forexample, support structure 102 could be used to maintain a physicalseparation between structures 110 and 120. As another example,electrically conductive structures 110 and 120 could each be attached toa ceramic package (not shown) that would keep the two electricallyconductive structures physically separate from each other.

As an example, the porous structure of electrically conductivestructures 110 and 120 can be created by a wet etch process in which aliquid etchant applied to a surface of the electrically conductivestructures etches away portions of the electrically conductive structurein a way that is at least somewhat similar to the way water is able tocarve channels in rock. This is why each one of the channels has anopening to the surface of the electrically conductive structure; the wetetch method is incapable of creating fully-enclosed cavities, i.e.,cavities with no opening to the surface, like an air bubble trappedinside a rock, within the porous structure. This is not to say thatthose openings cannot be covered with other materials or otherwiseclosed up because of the presence of or addition of other materials—thatis in fact likely to occur in several embodiments—but, whether coveredor not, the described openings to the surface are a feature of eachchannel in each porous structure according to at least one embodiment ofthe invention. (One embodiment in which the openings may be covered upis one in which a layer of epitaxial silicon as a location for circuitryor other wiring is grown on top of the channels).

With the right etchant, it should be possible to make porous structureshaving the described characteristics from a wide variety of materials.Silicon in various forms—including metallurgical grade silicon,monocrystalline silicon, polycrystalline silicon, and silicon oninsulator—is one material that works well. As an example, a poroussilicon structure may be created by etching a silicon substrate with amixture of hydrofluoric acid (HF) and ethanol or isopropyl alcohol. Moregenerally, porous silicon and other porous structures may be formed bysuch processes as anodization and stain etching. Etching techniquesaccording to embodiments of the invention will be discussed in moredetail below.

Besides porous silicon, which has already been mentioned, some othermaterials that may be especially well-suited for energy storage devicesaccording to embodiments of the invention are porous germanium andporous tin. Possible advantages of using porous silicon include itscompatibility with existing silicon technology and its abundance in theearth's crust. Porous germanium enjoys a similar advantage as a resultof existing technology for that material and, as compared to silicon,enjoys the further possible advantage that its native oxide (germaniumoxide) is water-soluble and so is easily removed. (The native oxide thatforms on the surface of silicon may trap charge, which is an undesirableresult.) Porous germanium is also highly compatible with silicontechnology. Possible advantages of using porous tin, which is azero-band-gap material, include its enhanced conductivity with respectto certain other conductive and semiconductive materials.

Other materials may also be used for the porous structure, includingsemiconducting materials such as gallium arsenide (GaAs), indiumphosphide (InP), boron nitride (BN), silicon carbide (SiC), and alloyssuch as an alloy of silicon and germanium. Organic semiconductors mayalso be used. In some embodiments the semiconducting materials—or eveninsulating materials—may be treated to make them electrically conductive(or more highly conductive). An example is silicon that is degeneratelydoped with boron. In addition to porous semiconducting substrates,porous conducting substrates may also be used for ECs, including, incertain embodiments, substrates composed of carbon or of metals such ascopper, aluminum, nickel, calcium, tungsten, molybdenum, and manganese.

The etching used to make the porous structures may be accomplished usingan electrochemical etch that makes use of a dilute mixture of HF andisopropyl alcohol (or ethanol or the like) to form nanometer pores thatcan extend through a significant portion of the substrate. As anexample, a porous structure such as porous semiconducting structure 510may be prepared by applying an electrochemical etch technique to a solidsilicon wafer having an initial resistivity of 0.7 milli-ohm centimeters(mΩ-cm) using as the etchant one of the HF mixtures referred to above. Acurrent density in a range of approximately 25 milliamps per squarecentimeter (mA/cm²) to 500 mA/cm² may be used. (The area component inthese values refers to an area of the substrate surface before formationof the pores.)

The foregoing discussion has made reference to porous structuresaccording to embodiments of the invention. These porous structures, asmentioned, can be formed within a variety of materials, includingsilicon (in various forms, including metallurgical grade silicon,monocrystalline silicon, polycrystalline silicon, and silicon oninsulator), germanium, GaAs, InP, BN, CdTe, tin, copper, aluminum,nickel, calcium, tungsten, molybdenum, manganese, silicon carbide,organic semiconductors, and silicon-germanium alloys. The material fromwhich the porous structure is made can, in at least some embodiments, bedoped with elements that increase its conductivity; this may be doneusing standard techniques that are known in the art. In one embodiment,the material in which the porous structure is formed is silicon and thedopant species is boron, which may be introduced into the silicon in aconcentration of, for example, 10¹⁹ atoms/cm³. Other possible dopantsinclude phosphorus and arsenic (though these and other n-type dopantsrequire an illumination process during etching that p-type dopants donot).

Embodiments of the invention that rely on electrochemical etching as thechannel creation technique have another reason for introducing dopantsinto the material from which the porous structure is to be made. Wheresilicon and an HF etchant are involved, it is thought that a highelectric field attracts holes at defects and at the tip of the poresthat aid the reaction between the silicon and the fluorine from theetchant. It is thought that the process involves the formation of SiF₄molecules in liquid form. The SiF₄ gets pulled away and eventually getswashed out of the channels, leaving hydrogen atoms that bond to thesidewalls and also form H₂ that then bubbles away as a gas. Somehydrogen atoms remain; these bond with remaining silicon atoms. Thisprocess etches the channel (anisotropically) downward as opposed toexpanding laterally in an isotropic manner (which would simply polishthe surface without forming channels). Additional details, as bestunderstood, are set forth below (though it must be said that precisedetails of the mechanism of porous silicon formation remain at leastsomewhat unclear).

In general terms, during channel formation, direct dissolution of thesemiconductor almost always competes with oxidation plus subsequentdissolution of the oxide. The etchant (e.g., HF), therefore, has to beable to dissolve the oxide. A second prerequisite for the dissolutionreaction and thereby channel formation in a semiconductor is theavailability of electronic holes. The silicon surface, in contact withaqueous HF solutions, becomes saturated by hydrogen, depleted ofelectronic holes, and tends to be chemically inactive with respect tothe electrolyte (this protects the channel sidewalls during the etchingprocess). If a voltage is applied to the electrodes, the holes presentin a silicon wafer start migrating towards the silicon-electrolyteinterface. At the interface, a hole removes one silicon bond and therebymakes one silicon atom more susceptible for interactions with theelectrolyte. Eventually, the silicon atom is transferred into thesolution. The electrode decomposes into areas with optimal currentdensity and channels are formed in areas with almost no current density.According to different models, initiation of the channel growth couldbegin at micro-cavities, structural defects, mechanically strainedareas, or local perturbation of the surface potential field.

Referring again to FIGS. 1 and 2, energy storage structure 100 furthercomprises (in the embodiment illustrated in FIG. 1) an electricallyconductive coating 140 on at least a portion of the porous structure andin at least some of channels 111 and/or channels 121. Such anelectrically conductive coating may be necessary in order to maintain orenhance the conductivity of the porous structure, which can also lowerthe ESR, thereby improving performance. For example, a device havinglower ESR is able to deliver higher power (which may be manifested interms of greater acceleration, more horse power, etc.). In contrast,higher ESR (a condition that often prevails inside a typical battery)limits the amount of available energy, at least partially due to thefact that much of the energy is wasted as heat, which is a keyconsideration for both long-term performance and safety.

Illustrated in FIGS. 1 and 2 is an electrolyte 150, which gives rise tothe EDL, as explained above. Electrolyte 150 (as well as the otherelectrolytes described herein) is represented in the drawings using arandom arrangement of circles. This representation is intended to conveythe idea that the electrolyte is a substance (liquid or solid, includinggel-like materials) containing free ionic charge carriers. The circleswere chosen for convenience and are not intended to imply any limitationas to the electrolyte components or qualities, including any limitationwith respect to the size, shape, or number of the ionic charge carriers.

FIG. 2B is a cross-sectional view of an energy storage structureaccording to embodiments of the invention. In a manner substantiallysimilar to the depiction in FIGS. 1 and 2 and the descriptions herein,energy storage structure 100 may comprise energy storage device 101 andnon-conductive support structure 103. Energy storage device 101 maycomprise electrically conductive structure 110 and electricallyconductive structure 120, which may be separated from each other bytrench 231 containing separator 130. Meanwhile, electrically conductivestructure 110 and electrically conductive structure 120 may be twoporous structures. Electrically conductive structure 110 may comprisechannels 111, and electrically conductive structure 120 may comprisechannels 121.

Trench 231 may separate electrically conductive structure 110 andelectrically conductive structure 120 using a complex shape such as themeandering space between two interdigitated electrodes. Accordingly,electrically conductive structure 110 may comprise a first electrode 116having a first plurality of finger portions 117, electrically conductivestructure 120 may comprise a second electrode 126 having a secondplurality of finger portions 127, and first plurality of finger portions117 and second plurality of finger portions 127 may be separated bytrench 231 having a shape of a meandering space between twointerdigitated electrodes. First electrode 116 may have a first sidewall118 (facing second electrode 126) and second electrode 126 may have asecond sidewall 128 (facing first electrode 116). At least a portion offirst sidewall 118 of first electrode 116 may have a first plurality ofpores 119, and at least a portion of second sidewall 128 of secondelectrode 126 may have a second plurality of pores 129

After the introduction of electrolyte 150, an electric double layer isformed within the channels of the porous structure, as depictedschematically in FIG. 3. In that figure, an electric double layer 330has been formed within one of channels 111. EDL 330 is made up of twocomponents: the electrical charge of the sidewalls of channel 111(depicted as being positive in FIG. 3 but which in other embodimentscould be negative); and the free ionic charge carriers in theelectrolyte. EDL 330 thus provides a separation of charge that isnecessary in order for the capacitor to function. As explained earlier,the large capacitance, and, hence, energy storage potential, of EDLcapacitors arises in part due to the small (approximately 1 nanometer(nm)) separation distance between electrolyte ionic charge carriers andthe electrode surface charge.

It should be noted that the FIG. 1 and FIG. 2 depictions of the porousstructures are highly idealized in that, to mention just one example,all of channels 111 and 121 are shown as only extending vertically. Inreality the channels may branch off in multiple directions to create atangled, disordered pattern that may look something like the porousstructure shown in FIGS. 4a and 4 b.

FIGS. 4a and 4b are scanning electron microscope (SEM) images of,respectively, a surface and a cross-sectional slice of a porousstructure 400 (in this case porous silicon). As illustrated, porousstructure 400 contains multiple channels 411. It should be understoodthat channels 411 are likely to twist and turn along their lengths suchthat a single channel may have both vertical and horizontal portions aswell as portions that are neither completely vertical nor completelyhorizontal but fall somewhere in between. Note that in FIG. 4b , thechannels extend near to but do not quite reach a bottom of the etchedstructure, thus leaving a layer 402 of un-etched silicon underneath thechannels. In one embodiment, un-etched layer 402 acts as a supportstructure for porous structure 400 (and for the corresponding energystorage device, not shown), and is thus the equivalent of supportstructure 102.

As was mentioned above, the energy storage mechanism for EDLCs isgenerally due to the formation of a charge double-layer at theelectrode/electrolyte interface when a voltage is applied to the system.Embodiments of the invention seek to improve energy storage capabilityeven beyond what an EDLC can offer by beginning with an EDLC framework(e.g., using porous silicon or another porous semiconducting material asdescribed herein) and building onto this framework advancedfunctionality that enables substantially greater energy storagecapability. Other advantages are also possible. For example, theenhanced energy storage comes with high achievable power density ascompared to batteries and the like, such that electrochemical capacitorsare capable of providing high power for applications such as (to takeone example) turbo modes in microprocessors.

Embodiments of the invention utilize approaches such as alternativeionic species to enhance the amount of charge that can be stored in thedouble layer, and/or device architectures that rely on Faradaicreactions for charge storage in either the electrode or electrolyte. Asan example, various embodiments combine porous semiconducting structuresthat are compatible with silicon or other semiconductor processtechnology with pseudocapacitors or hybrid electrochemicalcapacitor-battery technology. Some embodiments involve the use of apseudocapacitive film that coats one or both of the porous electrodes.Others involve the use of an electrolyte containing metal ions that canbe exchanged between the surfaces (not the bulk) of two nanostructuredelectrodes. Other embodiments involve the use of electrolytes thattogether constitute a redox system such that charge can be stored in theelectrolyte solution using reversible redox chemistry. Theseembodiments, each of which may be used in conjunction with either orboth of the others, are described in more detail below.

FIG. 5 is a cross-sectional view of a portion of an energy storagedevice 500 according to an embodiment of the invention. Energy storagedevice 500 is similar to, and shares many characteristics with, energystorage structure 100 of FIGS. 1 and 2, but is shown at a larger scalein order to more clearly show certain structural details that will bediscussed below.

As illustrated in FIG. 5, energy storage device 500 includes a poroussemiconducting structure 510 (equivalent to electrically conductivestructure 110 of energy storage structure 100) and a poroussemiconducting structure 520 (equivalent to electrically conductivestructure 120 of energy storage structure 100) separated by a separator530 (equivalent to separator 130 of energy storage structure 100).Porous semiconducting structure 510 comprises a plurality of channels511 (only one of which is shown) that contain an electrolyte 514, andporous semiconducting structure 520 comprises a plurality of channels521 that contain an electrolyte 524.

One or both of electrolytes 514 and 524 can comprise a non-aqueouselectrolyte, which may be advantageous in certain embodiments becausenon-aqueous electrolytes can go to higher voltages and can thuscontribute to an increase in the amount of energy that may be stored byenergy storage device 500 (since the energy of a capacitor isproportional to the square of its voltage). Other types of electrolyte,including any of those disclosed above, may also be used.

In some embodiments, at least one of the first and second electrolytescontains a plurality of metal ions, such as ions of lithium (Li⁺),sodium (Na⁺), and/or fluorine (F) or the like (indicated in FIG. 5 bydots 516). This would enable non-Faradaic reactions that store energy onthe surface of high surface area architectures. The resulting device maybe envisioned as a combination of an EDLC and a metal-ion battery inthat the metal ions are captured on the surface (rather than in thebulk) of the material to store the charge. The metal ions migrate to thenanostructured cathode through the electrolyte, entering the pores andreaching the surfaces in the interior of the cathode without having toundergo solid-state intercalation. This enables much higher energydensities and specific energies than is true for EDLCs and also enablesvery high power densities (based on the ability of the high surface areaof the material to accommodate a significant number of metal ions(comparable to the bulk of a flat battery)) and gives the promise forsignificantly longer lifetimes approaching that of an EDLC (i.e.,hundreds of thousands to millions of cycles, assuming optimized deviceperformance) while retaining all the benefits of using porous silicon(or other materials as discussed herein) for the conventional EDLC.

In order to ensure ideal, long-term cycling behavior, the porouselectrode could in some embodiments be coated with a passivation barrier(further described below) in order to prevent metal intercalation and/orto protect the semiconductor material from oxidation. In the absence ofsuch a barrier, the electrode could end up being a metal intercalationanode with extremely low solid-state diffusion rates and poorcyclability. Storing lithium or other metal ions in the bulk of amaterial means that the ions must leave the interior of a cathode andeventually enter the bulk of an anode during recharge, and vice versaduring discharge. As further described below, the use of ALD as atechnique to treat the electrode surfaces could enable optimization ofsmall metal ion adsorption to maximize the energy density of such adevice.

Pseudocapacitance can also originate from redox reactions in theelectrolytic solution. Accordingly, in some embodiments electrolytes 514and 524 are selected such that, taken together, they comprise a redoxsystem, meaning that the electrolytes interact with each other in such away that electrons can be reversibly exchanged between them (thusallowing energy to be stored within the electrolytes). One example of aredox system involves a first electrolyte (i.e., either 514 or 524) thatis an alkali metal iodides aqueous solution and a second electrolytethat is a vanadyl solution.

Electrochemical capacitors can include both electric double layercapacitance and redox (or, more generally, electron transfer) reactions(pseudocapacitance) together as a charge storage mechanism. In order totake advantage of this, energy storage device 500 further comprises afilm 535 on at least one of porous semiconducting structures 510 and 520(in the illustrated embodiment the film is on both) comprising amaterial capable of exhibiting reversible electron transfer reactions.The presence of such a material makes energy storage device 500 apseudocapacitor. Film 535 acts to store charge in reversible faradicreactions between different oxidation states of the crystallinedeposited material.

As mentioned above, pseudocapacitive materials store energy throughhighly reversible surface electron transfer reactions. If such materialsare placed onto an EDLC, the resulting pseudocapacitor benefits fromboth types of energy storage—the electric double-layer storage inaddition to the electron transfer storage. This combination yieldsadvantages of both types of devices. For example, the highly reversibleelectrostatic charge storage in EDLCs does not produce the changes involume that usually accompany the redox reactions of the active massesin batteries, as the charge is stored on the surface. Furthermore, inpseudocapacitors the volume change is small since the active materialsare coated in thin crystalline films on high surface-area structures.Such volume changes in bulk electrode systems are the main cause of thelimited cycle life of batteries (generally several hundred to a fewthousand cycles) due to difficulties in charge transport across cracksthat develop to relieve the stress in bulk crystalline films after amoderate number of charge-discharge cycles. The EDLC and pseudocapacitorsystems of embodiments of the invention enable expected devicecyclability in the hundred thousand to million cycle range.

In some embodiments, the material of film 535 is a conducting polymersuch as polypyrrole, polyaniline, polythiophene, or the like. In otherembodiments, the material may comprise a nitride, a carbide, an oxide, asilicide, a boride, a borosilicide, a phosphide, a borophosphide, asulfide, or the like. Nitrides may include nitrides of titanium,vanadium, niobium, tantalum, molybdenum, tungsten, chromium,vanadium-titanium, and titanium-aluminum. Carbides may include carbidesof titanium, zirconium, hafnium, vanadium, niobium, tantalum,molybdenum, tungsten and any other conductive metal nitride or carbide,including carbonitrides. Silicides and borosilicides may includesilicides or borosilicides of titanium, zirconium, hafnium, nickel, andcobalt. Phosphides and borophosphides may include phosphides orborophosphides of iron, ruthenium, cobalt, and nickel. Oxides mayinclude oxides of ruthenium, lead, iridium, cobalt, manganese,molybdenum, tungsten, nickel, niobium, vanadium, chromium, andcarbon/transition metal composites. In each case, the poroussemiconducting structures with which film 535 is associated may beformed in silicon or any other suitable material, including any of thesemiconducting (or other) materials disclosed herein.

Transition metal nitrides and carbides are promising candidates aselectrode materials for electrochemical capacitors due to their highelectronic conductivities and electrochemical stability in aqueouselectrolytes. Higher capacitance is achieved by a series of reversibleredox reactions through hydroxy bonding confined to a few atomic layersof oxide on the surface of an underlying conductive nitride. VN in KOHand Mo₂N in H₂SO₄ show particular promise due to their relatively largestability windows of 1.2V and 0.8V (see Table 1). Vanadium exhibitsnumerous oxidation states and can be deposited using vanadium amideTDEAV [tetrakis(diethylamido)vanadium] and ammonia as a precursor withALD. For high aspect ratios, a high vapor pressure precursor such asVCl₄ (e.g., with a vapor pressure of 6 torr at 20° C.) would be morepreferable with NH₃ as a co-reactant.

TABLE 11 Stability Window (V) Material KOH H₂SO₄ VN 1.2 Unstable VC 0.7Unstable Mo₂N Unstable 0.8 Mo₂C Unstable Unstable W₂N 0.7 Unstable W₂CUnstable 0.7

In certain embodiments, film 535 may be on the order of one or twonanometers thick. Any of the materials mentioned above may be depositedusing ALD processes, which are capable of depositing at the giventhickness scale. Metal silicides could also be prepared by solution“electroless” metal deposition of Ni, Co etc. followed by anneal.

Referring still to FIG. 5, energy storage device 500 may furthercomprise a passivation barrier 540 that coats semiconducting structuresassociated with electrolytes that contain metal ions (e.g., either oneof or both of porous semiconducting structures 510 and 520) in order toprevent metal intercalation (i.e., the insertion of metal molecules intothe molecular structure of electrodes 510 and 520). As an example,barrier 540 may be similar to electrically conductive coating 140 thatwas introduced above and shown in FIG. 1. In some embodiments, barrier540 is part of film 535 such that film 535 exhibits a bilayer structurewith the pseudocapacitive film as an upper layer and barrier 540 as alower layer that lies between the pseudocapacitive film and the poroussemiconducting structures. In other embodiments, barrier 540 constitutesa separate layer that is distinct from (but still adjacent to) film 535.In various embodiments, barrier 540 can comprise an electricallyconducting material such as TiN, ruthenium, tungsten, copper, and thelike as well as an electrically insulating material such as a high-koxide or the like (where “high-k” means a dielectric constant greaterthan 3.9). Any of the foregoing materials (with the possible exceptionof copper, which suffers from a lack of high-aspect-ratio ALD processes)may be deposited using ALD. In certain embodiments, a conductivematerial for barrier 540 may be advantageous because it greatly enhancesthe collection of electrons involved in the pseudocapacitive material'selectron transfer reactions.

FIG. 6 is a schematic representation of a mobile electronic device 600according to an embodiment of the invention. As illustrated in FIG. 6,mobile electronic device 600 comprises a housing 601, an integratedcircuit (IC) die 610 within the housing, and an energy storage device620 within housing 601 and associated with IC die 610 so as to becapable of providing energy to the IC die. In some embodiments, being“associated with” IC die 610 means energy storage device 620 isintegrated into IC die 610 or its packaging in some fashion (e.g., bybeing implemented on the die itself; by forming part of aPackage-on-Package (PoP) architecture or a system-on-chip (SoC)architecture; etc.)

IC die 610 may comprise any type of integrated circuit device. In oneembodiment, the IC die includes a processing system (either single coreor multi-core). For example, the IC die may comprise a microprocessor, agraphics processor, a signal processor, a network processor, a chipset,etc. In one embodiment, IC die 610 comprises an SoC having multiplefunctional units (e.g., one or more processing units, one or moregraphics units, one or more communications units, one or more signalprocessing units, one or more security units, etc.). However, it shouldbe understood that the disclosure is not limited to any particular typeor class of IC device. If a PoP assembly is used it may comprise part ofany type of computing system, as described below.

Energy storage devices as disclosed herein may be used as part ofcomputing systems of various kinds. One example of such a computingsystem is the mobile electronic device described above. In that or otherembodiments, the computing system may include a number of componentsdisposed on a mainboard or other circuit board. The mainboard mayinclude a first side and an opposing second side, and various componentsmay be disposed on either one or both of the first and second sides. Forexample, the computing system may include an energy storage device suchas disclosed herein disposed on the mainboard's first or second side.The system may comprise any type of computing system, such as, forexample, a hand-held or mobile computing device (e.g., a cell phone, asmart phone, a mobile internet device, a music player, a tabletcomputer, a laptop computer, a nettop computer, etc.). However, thedisclosed embodiments are not limited to hand-held and other mobilecomputing devices and these embodiments may find application in othertypes of computing systems, such as desk-top computers and servers.

The mainboard may comprise any suitable type of circuit board or othersubstrate capable of providing electrical communication between one ormore of the various components disposed on the board. In one embodiment,for example, the mainboard comprises a printed circuit board (PCB)comprising multiple metal layers separated from one another by a layerof dielectric material and interconnected by electrically conductivevias. Any one or more of the metal layers may be formed in a desiredcircuit pattern to route—perhaps in conjunction with other metallayers—electrical signals between the components coupled with the board.However, it should be understood that the disclosed embodiments are notlimited to the above-described PCB and, further, that the mainboard maycomprise any other suitable substrate.

In addition to one or more energy storage devices, one or moreadditional components may be disposed on either one or both sides of themainboard. Additional components that may be disposed on the mainboardinclude other IC devices (e.g., processing devices, memory devices,signal processing devices, wireless communication devices, graphicscontrollers and/or drivers, audio processors and/or controllers, etc.),power delivery components (e.g., a voltage regulator and/or other powermanagement devices, a power supply such as a battery, and/or passivedevices such as a capacitor), and one or more user interface devices(e.g., an audio input device, an audio output device, a keypad or otherdata entry device such as a touch screen display, and/or a graphicsdisplay, etc.), as well as any combination of these and/or otherdevices. In some embodiments the computing system may include one ormore of a radiation shield, a cooling solution, an antenna, or the like.In some embodiments the computer system may be disposed within a housingor case. Where the mainboard is disposed within a housing, some of thecomponents of the computer system—e.g., a user interface device, such asa display or keypad, and/or a power supply, such as a battery—may beelectrically coupled with the mainboard (and/or a component disposed onthis board) but may be mechanically coupled with the housing.

As an example, energy storage device 620 can be similar to energystorage device 500, a portion of which is shown in FIG. 5. Accordingly,energy storage device 620 is characterized by at least one of thefollowing statements (the details of each of which have been discussedabove): (1) the mobile electronic device further comprises a film on atleast one of the first and second porous semiconducting structures, thefilm comprising a material capable of exhibiting reversible electrontransfer reactions; (2) at least one of the first and secondelectrolytes contains a plurality of metal ions; and (3) the first andsecond electrolytes, taken together, comprise a redox system.

FIG. 7 is a flowchart illustrating a method 700 of manufacturing anenergy storage device according to an embodiment of the invention. As anexample, method 700 may result in the formation of an energy storagedevice similar to energy storage device 500, described above.

An operation 710 of method 700 is to provide a first poroussemiconducting structure comprising a first plurality of channels thatcontain a first electrolyte. As an example, the first poroussemiconducting structure can be similar to porous semiconductingstructure 510 that is shown in FIG. 5.

An operation 720 of method 700 is to provide a second poroussemiconducting structure comprising a second plurality of channels thatcontain a second electrolyte. As an example, the second poroussemiconducting structure can be similar to porous semiconductingstructure 520 that is shown in FIG. 5.

In certain embodiments, method 700 further comprises, either as part ofoperations 710 and 720 or as one or more separate operations, providingat least one of the first and second electrolytes with a plurality ofmetal ions and depositing a barrier layer on the at least one of thefirst and second porous semiconducting structures prior to depositingthe film. In the same or other embodiments, the first and secondelectrolytes may be selected such that, taken together, they comprise aredox system. As an example, the barrier layer can be similar topassivation barrier layer 540 that is shown in FIG. 5 and the metal ionscan be as described elsewhere herein. As has also been described, insome embodiments the barrier layer may be part of a bi-layer structurethat also includes the film deposited in operation 730 as discussedbelow. In such cases, the barrier layer would be deposited onto (orotherwise applied to) the porous electrode first after which thepseudocapacitive film would be deposited or otherwise applied. As hasbeen described above, in some embodiments depositing the barrier layercomprises using an ALD technique.

An operation 730 of method 700 is to deposit on at least one of thefirst and second porous semiconducting structures a film comprising amaterial capable of exhibiting reversible electron transfer reactions.In some embodiments, operation 730 comprises using an ALD technique. Asan example, the film can be similar to film 535 that is shown in FIG. 5.

In a particular embodiment, the film comprises vanadium nitride (VN) andthe ALD technique is performed using TDEAV and ammonia (NH₃) or ahydrazine (e.g., N₂H₄, methylhydrazine, dimethylhydrazine,tert-butylhydrazine) as precursors. In a different embodiment, the filmcomprises vanadium nitride and the ALD technique is performed using VCl₄and NH₃ or a hydrazine as precursors. In yet another embodiment, thefilm comprises vanadium titanium nitride and the ALD technique isperformed using VCl₄, TiCl₄, and NH₃ or a hydrazine as precursors. Instill another embodiment, the film comprises RuO₂ and the ALD techniqueis performed using an organoruthenium compound (such asbis(ethylcyclopentadienyl)ruthenium) and oxygen as precursors.

FIG. 8 is a flowchart illustrating a method 800 of manufacturing anenergy storage device according to another embodiment of the invention.As is the case with method 700 above, method 800 may result in theformation of an energy storage device similar to energy storage device500.

An operation 810 of method 800 is to provide a first porous structurecomprising a first plurality of channels that contain a firstelectrolyte.

An operation 820 of method 800 is to provide a second porous structurecomprising a second plurality of channels that contain a secondelectrolyte.

In certain embodiments, method 800 further comprises, either as part ofoperations 810 and 820 or as one or more separate operations, providingat least one of the first and second electrolytes with a plurality ofmetal ions and depositing a barrier layer on the at least one of thefirst and second porous semiconducting structures prior to depositingthe film. In the same or other embodiments, the first and secondelectrolytes may be selected such that, taken together, they comprise aredox system. As an example, the barrier layer can be similar topassivation barrier layer 540 that is shown in FIG. 5 and the metal ionscan be as described elsewhere herein. As has also been described, insome embodiments the barrier layer may be part of a bi-layer structurethat also includes the film deposited in operation 830 as discussedbelow. In such cases, the barrier layer would be deposited onto (orotherwise applied to) the porous electrode first after which thepseudocapacitive film would be deposited or otherwise applied. As hasbeen described above, in some embodiments depositing the barrier layercomprises using an ALD technique.

An operation 830 of method 800 is to use an ALD technique to deposit onat least one of the first and second porous structures a film comprisinga material capable of exhibiting reversible electron transfer reactions.As an example, the film can be similar to film 535 that is shown in FIG.5.

Although the invention has been described with reference to specificembodiments, it will be understood by those skilled in the art thatvarious changes may be made without departing from the spirit or scopeof the invention. Accordingly, the disclosure of embodiments of theinvention is intended to be illustrative of the scope of the inventionand is not intended to be limiting. It is intended that the scope of theinvention shall be limited only to the extent required by the appendedclaims. For example, to one of ordinary skill in the art, it will bereadily apparent that the energy storage device and the relatedstructures and methods discussed herein may be implemented in a varietyof embodiments, and that the foregoing discussion of certain of theseembodiments does not necessarily represent a complete description of allpossible embodiments.

Additionally, benefits, other advantages, and solutions to problems havebeen described with regard to specific embodiments. The benefits,advantages, solutions to problems, and any element or elements that maycause any benefit, advantage, or solution to occur or become morepronounced, however, are not to be construed as critical, required, oressential features or elements of any or all of the claims.

Moreover, embodiments and limitations disclosed herein are not dedicatedto the public under the doctrine of dedication if the embodiments and/orlimitations: (1) are not expressly claimed in the claims; and (2) are orare potentially equivalents of express elements and/or limitations inthe claims under the doctrine of equivalents.

What is claimed is:
 1. An apparatus comprising: a first electrodecomprising a silicon material and having a first plurality of fingerportions; and a second electrode comprising the silicon material andhaving a second plurality of finger portions, wherein the firstplurality of finger portions and the second plurality of finger portionsare interdigitated and are separated by a trench; wherein the firstelectrode has a first sidewall facing the second electrode, the firstsidewall including a first sidewall portion having a first plurality ofpores and a conductive coating; and wherein the second electrode has asecond sidewall facing the first electrode, the second sidewallincluding a second sidewall portion having a second plurality of poresand a conductive coating.
 2. The apparatus of claim 1, wherein theconductive coating of the first sidewall portion comprises one of:titanium nitride, titanium carbide, titanium silicide, and titaniumborosilicide.
 3. The apparatus of claim 1, wherein the conductivecoating of the first sidewall portion comprises a nitride of one of:titanium, vanadium, niobium, tantalum, molybdenum, tungsten, chromium,vanadium-titanium, or titanium-aluminum.
 4. The apparatus of claim 1,wherein the conductive coating of the first sidewall portion coatsinterior surfaces of the first plurality of pores.
 5. The apparatus ofclaim 1, wherein the first plurality of pores and the second pluralityof pores contain an electrolyte.
 6. The apparatus of claim 1, whereinthe conductive coating of the first sidewall portion is a thin filmformed by atomic layer deposition.
 7. The apparatus of claim 1, whereinthe first plurality of pores and the second plurality of pores areformed by an etch of the first sidewall surface and the second sidewallsurface.
 8. The apparatus of claim 1, wherein the first electrodeincludes a first portion of non-porous silicon and the second electrodeincludes a second portion of non-porous silicon.
 9. The apparatus ofclaim 1, comprising: a support structure to maintain a physicalseparation between the first electrode and the second electrode.
 10. Theapparatus of claim 1, wherein the first electrode and the secondelectrode are implemented in a single integrated circuit.
 11. Theapparatus of claim 1, wherein the silicon material comprises at leastone of: phosphorous, arsenic, and boron.
 12. A mobile electronic devicecomprising a housing, an integrated circuit die within the housing, andan energy storage device within the housing and associated with theintegrated circuit die so as to be capable of providing energy to theintegrated circuit die, wherein the energy storage device comprises: afirst electrode comprising a silicon material and having a firstplurality of finger portions; and a second electrode comprising thesilicon material and having a second plurality of finger portions,wherein the first plurality of finger portions and the second pluralityof finger portions are interdigitated and are separated by a trench;wherein the first electrode has a first sidewall facing the secondelectrode, the first sidewall including a first sidewall portion havinga first plurality of pores and a conductive coating; and wherein thesecond electrode has a second sidewall facing the first electrode, thesecond sidewall including a second sidewall portion having a secondplurality of pores and a conductive coating.
 13. The mobile electronicdevice of claim 12, wherein the conductive coating of the first sidewallportion comprises titanium nitride (TiN); and wherein the siliconmaterial comprises at least one of: phosphorous, arsenic, and boron. 14.The mobile electronic device of claim 12, wherein the conductive coatingof the first sidewall portion coats interior surfaces of the firstplurality of pores; and wherein the first plurality of pores and thesecond plurality of pores contain an electrolyte.
 15. The mobileelectronic device of claim 12, wherein the first plurality of pores andthe second plurality of pores contain an electrolyte.
 16. The mobileelectronic device of claim 12, wherein the conductive coating of thefirst sidewall portion is a thin film formed by atomic layer deposition;and wherein the first plurality of pores and the second plurality ofpores are formed by an etch of the first sidewall surface and the secondsidewall surface.
 17. A method of manufacturing an energy storagedevice, the method comprising: providing a first silicon structurehaving a first plurality of finger portions and a second siliconstructure having a second plurality of finger portions, the firstplurality of finger portions and the second plurality of finger portionshaving an interdigitated configuration; spacing the first siliconstructure from the second silicon structure by a trench positionedbetween the first plurality of finger portions and the second pluralityof finger portions; etching a side surface of the first siliconstructure that faces the second silicon structure to create a pluralityof pores; etching a side surface of the second silicon structure thatfaces the first silicon structure to create a plurality of pores; andcoating the side surfaces and the pluralities of pores of the firstsilicon structure and the second silicon structure with Titanium Nitride(TiN).
 18. The method of claim 17, comprising: providing the pluralitiesof pores of the first silicon structure and the second silicon structurewith an electrolyte comprising at least one of: a solid substance, and agel-like material.
 19. The method of claim 17, wherein the side surfacesand the pluralities of pores are coated with TiN by atomic layerdeposition.
 20. The method of claim 17, comprising: providing the energystorage device with a support structure to maintain a physicalseparation between the first silicon structure and the second siliconstructure; and integrating the first silicon structure and the secondsilicon structure into an integrated circuit die.